Sky Glow: A Case of the Blues
Research reveals why spectrum alone is not enough to evaluate sky glow
An increasing number of cities are undertaking street light conversion projects—switching from technologies such as high-pressure sodium (HPS) luminaires to energy-efficient light-emitting diodes (LEDs). However, as broad-spectrum or "white" light sources, LEDs have significantly higher short wavelength content than the HPS luminaires they are replacing. Given short wavelengths scatter more readily in Earth’s atmosphere than long wavelengths, dark-sky advocates have voiced concerns about possible increases in sky glow resulting from ongoing LED conversions.
Sky glow (noun): an increase in the brightness of the night sky due to anthropogenic illumination of the night environment
Sky glow is of concern because it obscures visibility of celestial objects, such as stars. But are LED street lights the primary culprit?
Researchers at PNNL explored issues surrounding LED street lights and sky glow, publishing their findings in the report, An Investigation of LED Street Lighting's Impact on Sky Glow. The researchers ultimately found that, due to additional factors that tend to collectively characterize the typical conversion to LEDs, residents within a city are not likely to experience worsened sky glow as a result of LEDs and, in fact, will possibly realize an improved situation. What’s more, areas outside of cities are likely to be much better off when the city uses modern luminaires.
Bruce Kinzey and his team noted that the current debate tends to neglect these other factors that are critical contributors to sky glow. “There are multiple characteristics that influence sky glow,” Kinzey said. "In addition to spectrum, total lumen output and distribution of the light produced are key. Regarding the latter, of principal importance is the amount of light emitted upwards."
Sponsor & Partners
This research was funded by DOE's Solid-State Lighting Technology Program.
International research partners included the Slovak Academy of Sciences, Cégep de Sherbrooke, and the Interdisciplinary Center for Metropolitan Studies.
LED vs. HPS
The researchers compared the spectral contents of seven different LED sources, as well as low-pressure sodium and metal halide sources, with that of a baseline HPS light source. They also ran an equal energy spectrum in five-nanometer increments to test the sensitivity of the results to spectrum at a granular level. Five different sets of atmospheric conditions were run, four of them representing clear skies with increasing aerosol contents—such as dust, smog, or water droplets—and the fifth condition overcast with cloud cover. Three city sizes were modeled from two different observer positions, one at the perimeter of each city and the other about 25 miles from the city center. Finally, aspects of HPS and LED luminaires were varied to determine the impacts of light output, in lumens, and percent uplight.
Given the number of variables and iterations required to represent all combinations, the entire set amounted to more than 200,000 simulations run.
Ultimately, numerous factors have at least some influence on the amount of sky glow present at a given location. "Our research shows that even considering spectrum by itself, much less only the apparent color of a light source, isn’t nearly enough to identify possible sky glow contributions," said Kinzey. "Sky glow is a real issue that has been growing for decades, but many factors contribute to it. The good news is we’re addressing some of the primary contributors from street lighting as part of the typical conversion to LEDs that’s already taking place."
Math & Models
To complete the large number of simulation runs required in a reasonable timeframe, Kinzey and his team partnered with PNNL’s Institutional Computing program to model sky glow, leveraging one of the Lab's Windows HPC clusters.
The model used—SkyGlow Simulator—was originally developed in 2007 by Miroslav Kocifaj. SkyGlow Simulator characterizes the impact on the sky glow dome with respect to the observer and ultimately provides a variety of metrics, namely diffuse irradiance or illuminance on the horizontal plane.
Kinzey also stressed that although his team didn't model other sources of light at night beyond street lights, considering other contributors—such as commercial office building interior lighting that escapes via exterior windows, signage and landscape lighting, and sports lighting—is crucial if identifying and reducing sky glow is something society is serious about.
For more information, download the report on DOE’s website.
Research Team: Bruce Kinzey, Tess Perrin, and Naomi Miller